I think he means that, in the event of a break-in, any documents that have personal data, or that could be easily used to forge a false title or registration, could bite you.
Or leaving house keys in the car, or even the garage remote (it's not hard to open those, and find out the code; now we have an address, plus the code for the garage door opener, or a spare key.)
He has a point; this is why keeping the title in the car is a monumentally bad idea. For that matter, so is keeping spare house keys.
However, I am willing to run the risk with title and registration.